1. Field of the Invention
The present invention is directed to a method and apparatus for making an optical fiber preform and, more particularly, to a method and apparatus for making an optical fiber preform using a plasma torch and including one or more steps of substantially simultaneous inside deposition and/or consolidation and outside deposition and/or consolidation.
2. Statement of the Problem
Various methods and apparatus for making optical fiber preforms are known in the optical fiber industry and described in its related publications. For example, U.S. Pat. No. 6,253,580 (xe2x80x9cthe ""580 patentxe2x80x9d) describes a Plasma Outside Vapor Deposition (xe2x80x9cPOVDxe2x80x9d) process for making synthetic silica tubes. The synthetic silica tubes made in accordance with the ""580 invention can be used as a substrate or as a jacketing tube in fabricating optical fiber preforms by the Modified Chemical Vapor Deposition (xe2x80x9cMCVDxe2x80x9d) methods. Further, the processing rate and quality of synthetic silica tubes made in accordance with the ""580 patent is favorable based on many of the presently established criteria. However, improved processing rate and quality are always desirable. Cost, though, is another factor that must always be considered.
The prior art shows numerous methods for making preforms and other fiber-related glass and silica products. These include the MCVD process such as disclosed by, for example, U.S. Pat. No. 3,982,916 to Miller and Pat. No. 4,217,027 to MacChesney. These also include the Plasma Chemical Vapor Deposition process such as disclosed by, for example, U.S. Pat. Nos. 4,741,747 and 4,857,091, both to Geittner et al. Further included is the MCVD with radio frequency (xe2x80x9crfxe2x80x9d) plasma process such as disclosed by, for example, U.S. Pat. No. 4,262,035 to Jaeger et al. and Pat. No. 4,331,462 to Fleming et al., and the method of MCVD with a plasma torch such as disclosed by, for example, U.S. Pat. Nos. 5,397,372 and 5,692,087, both to Partus et al.
The present inventors have identified that the processes and methods disclosed by the above-listed patents have various shortcomings with respect to current and future requirements for production rate and fiber quality.
Other methods known in the prior art include the Outside Vapor Deposition Process (xe2x80x9cOVDxe2x80x9d) disclosed by U.S. Pat. No. 3,737,292 to Keck and U.S. Pat. No. 3,932,162 to Blakenship, and the Vapor Axial Deposition (xe2x80x9cVADxe2x80x9d) process disclosed by, for example, U.S. Pat. Nos. 4,062,665 and 4,224,04, both to Izawa et al. The present inventors, though, have identified that the processes and methods as disclosed by the above-listed patents have various shortcomings including, for example, the necessity for performing separate steps for sintering or consolidation of the deposited silica.
Still other known methods for making preforms include the method of sleeving and collapsing a tube or tubes on a primary preform using, for example, a plasma torch, as disclosed by U.S. Pat. No. 5,578,106 to Fleming et al., or an oxygen-hydrogen torch, as disclosed by U.S. Pat. No. 4,596,589 to Perry and U.S. Pat. No. 4,820,322 to Baumgart. The present inventors have identified shortcomings with all of these methods, including, for example, the requirement for a jacketing process.
The prior art also includes the overcladding process as disclosed by U.S. Pat. No. 5,522,007 to Drourt. These methods include the steps of building up a large diameter preform by depositing cladding glass onto a primary preform, using a plasma torch. A typical shortcoming of overcladding is its necessary addition of one or more additional steps, namely that the primary preform be made first, followed by adding the overcladding layers, which adds time and equipment costs.
It is known in the optical fiber industry that one method for lowering cost, and for increasing processing rate, is to make larger preforms. For example, as reported by Glodis et al. in U.S. Pat. No. 6,105,396 (xe2x80x9cthe ""396 patentxe2x80x9d), a preform can be made which generates approximately 400 kilometers of fiber.
The benefits of making larger preforms manifest in at least two stages, or steps, of manufacturing fiberxe2x80x94the preform fabrication step and the fiber draw step. Regarding the first step, the immediate benefit that is seen from using a larger preform is that the larger the preform the greater the length of fiber that it produces.
For example, the set-up and inspection time for making the larger preform should not be substantially longer than the set-up and inspection time for making smaller preforms. This is an important consideration because the initial set-up for fabricating a preform, together with the post-processing inspections, occupy a significant percentage of the time required to fabricate a preform. Therefore, more net increase in manufacturing efficiency is gained when using a larger preform if the set-up and inspection times during its manufacture are kept substantially the same as those for a smaller preform.
Improved fiber quality is another benefit gained by using a larger preform. This is because drawing fiber is basically a stretching of the preform volume. A larger diameter preform has a greater volume per unit length and, therefore, when compared to a smaller diameter preform, a shorter linear section of the preform is required to form the same length of fiber. The optical qualities of a preform typically vary along its length. Therefore, since the larger preform requires less length to produce a given length of fiber, fiber drawn from it has a correspondingly lower rate of variation per unit length than would be seen in fiber drawn from a smaller preform.
There are other objectives that must be met when fabricating larger preforms so that the larger size provides a practical, usable increase in manufacturing efficiency. Low equipment cost is one of these objectives. Namely, the decrease in cost that can be obtained by fabricating larger preforms will be maximized by a method that requires minimum purchase and installation of new equipment.
Another problem relating to MCVD processes, and to making larger preforms, is the incomplete oxidation of dopants flowing through the hollow or void in the tube. A reason is that the base glass chemical, such as SiCl4, and the dopant chemicals, such as GeCl4, POCl3 and SF6, flow together into the hollow. Because of the plurality of reactants present, there are multiple chemical reactions that result. A typical effect of the multiple reactions is that only one is essentially completed, this frequently being the reaction of SiCl4 vapors with O2. In contrast, the dopant oxidation reactions are frequently not complete. For example, in the conventional MVCD manufacturing of germanium doped silica, a large fraction of the dopant appears in the gaseous effluent in the form of GeCl4. Published reports such as xe2x80x9cGermanium Chemistry in the MCVD Process for Optical Fiber Fabrication,xe2x80x9d J. of Lightwave Technology, LT-5, no.2, 1987, 277-285, show that as much as 70% of the initial germanium flowing into the hollow is present in the effluent as GeCl4.
The present invention provides an apparatus and method for making an optical fiber preform using plasma deposition on a silica tube, where at least a portion of the process performs concurrent deposition, consolidation, or deposition/consolidation of silica on the inside and the outside of the tube. Certain steps within the described embodiments deposit soot, without consolidation, on one of the inner and outer surfaces of the tube, concurrent with simultaneous deposition and consolidation of soot on the other of the inner and outer surface. Other steps perform concurrent deposition, without consolidation, followed by concurrent consolidation, with or without deposition of additional soot during the concurrent consolidation pass. The total deposition rate is increased over the prior art due to the concurrent deposition of soot on the inner and outer surfaces of the tube. The deposition rate is also increased by the invention setting the rate of traversing the plasma flame in accordance with the desired concurrent inner and outer surface deposition and the desired inner and outer surface consolidation.
The apparatus and method of the present invention achieves this simultaneous formation of silica layers on the inside and outside of the tube by various multistep methods that traverse a plasma along a rotating tube, selectively injecting reaction-producing chemicals into the hollow, or void of the tube, while selectively injecting other reaction-producing chemicals into the plasma. The chemicals injected into the hollow, collectively referenced as xe2x80x9cCFITxe2x80x9d, and the chemicals injected into the plasma, collectively referenced as xe2x80x9cCFOTxe2x80x9d, chemicals undergo chemical reactions due to the heat from the plasma torch. The reactions cause one or more of the following operations on the inside and/or outside of the tube: a deposition of soot, a consolidation of a soot previously deposited to form a vitreous silica layer, or a simultaneous deposition of soot and consolidation of same into a vitreous silica layer. The operations can be the same on the inside and the outside, or one may be performed on the inside while another is performed on the outside. The respective operations can be different as the plasma torch is traversed in one direction down the tube as compared to the operations performed as the plasma is traversed in the opposite direction. The vitreous silica layer can be either doped or undoped, depending on the makeup of the CFIT and CFOT chemicals. Accordingly, the phrases xe2x80x9cvitreous silica layerxe2x80x9d and xe2x80x9csilica layerxe2x80x9d are each defined to include both doped and undoped silica unless otherwise stated or clear from the context in which it is used.
An apparatus according to the invention includes a workpiece rotation apparatus such as, for example, a glassworking lathe, for securing and rotating a hollow tubular silica member about a center axis, a movable support for a plasma torch, a plasma torch having a plasma generating coil secured to the movable support for generating a plasma flame incident on an outer surface of the hollow tubular silica member, a first translation actuator for moving the movable support and the plasma torch at a selectable translation rate parallel to the center axis, a second translation actuator for selectively moving the movable support toward and away from the center axis to space the coil selectively with respect to an outer surface of the tubular member, a source chemical controller feeds for selectively injecting CFIT source chemicals into the hollow of the tubular member while the tubular member is rotating, and another source chemical controller feeds for selectively injecting CFOT source chemicals into the plasma flame generated by the plasma torch.
Optionally, a distance sensor is operatively connected to the second translation actuator, for detecting a distance between a reference point relative to the plasma torch and the outside surface of the tubular member and generating a distance signal based on the detected distance. A processor generates a translation control signal based on the distance signal and a predetermined distance value. The second translation actuator receives the translation control signal and moves in accordance with it.
One aspect of the invention is a method for making an optical fiber preform including steps of: (a) providing a silica tubular member, the silica tubular member having an interior surface surrounding a cylindrical void extending along a center axis and having an exterior surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) forming an outer vitreous silica layer on the outer surface of the tube concurrent with forming a selectively doped inner vitreous silica layer on the inner surface of the tube, (d) depositing an outer soot layer on the outer vitreous silica layer, (e) consolidating the outer soot layer deposited at step (d) into an outer vitreous silica layer concurrent with forming along the length of the tube an outer vitreous silica layer on the outer surface of the tube, concurrent with forming a selectively doped inner vitreous silica layer on the inner surface of the tube, and (f) repeating steps (d) and (e) until a predetermined thickness of inner vitreous silica layers is formed.
This aspect of the invention performs steps (c) and (e) by, for example, traversing a plasma flame in a first direction along the rotating tubular member at a first forward traversal rate TF1 while injecting CFIT chemicals into the cylindrical void to flow in the first direction and, concurrently, injecting CFOT chemicals into the plasma. For this example performance of steps (c) and (e), the generating of the plasma and the first traversal rate are such that the plasma deposits an outer layer of soot and consolidates that soot and, for step (e), also consolidates the soot deposited at step (d), into an outer vitreous silica layer in accordance with the CFOT chemicals. The generation of plasma and the first forward traversal rate are also such that substantially simultaneous to the deposition and consolidation of the outer vitreous silica layer, the plasma effects depositing of inner soot particles and concurrent consolidation of same into an inner layer of vitreous silica selectively doped in accordance with the CFIT chemicals.
The first aspect of the invention may perform step (d) by, for example, traversing the plasma at a first reverse traversal rate TR1 in a direction opposite the first direction, with no substantial CFIT chemicals flowing into the void, while injecting CFOT chemicals into the plasma, where the plasma, the CFOT chemicals and the first reverse traversal rate are such that a soot in accordance with the CFOT chemicals is deposited without substantial consolidation.
A second aspect of the invention is a method for making an optical fiber preform including steps of: (a) providing a silica tubular member, the silica tubular member having an interior surface surrounding a cylindrical void extending along a center axis and having an exterior surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) forming an outer vitreous silica layer along a length of the tube above the outer surface of the tube concurrent with forming a selectively doped inner vitreous silica layer along the length of the tube inward from the inner surface of the tube, (d) depositing an outer soot layer on the outer vitreous silica layer and, concurrent with depositing the outer soot layer, depositing a selectively dopes inner soot layer on the inner vitreous silica layer, (e) consolidating the outer soot layer deposited at step (d) concurrent with forming an outer vitreous silica layer on the outer surface of the tube, concurrent with consolidating the inner soot layer deposited at step (d) concurrent with forming a vitreous silica layer on the inner surface of the tube, and (f) repeating steps (d) and (e) until a predetermined thickness of vitreous silica layers is formed by the steps.
A method according to the second aspect of the invention may perform steps (c) and (e) by, for example, traversing a plasma flame in a first direction along the rotating tubular member at a second forward traversal rate TF2 while injecting CFIT chemicals into the cylindrical void to flow in the first direction and injecting CFOT chemicals into the plasma. The generation of the plasma and the second forward traversal rate for such an example performance of steps (c) and (e) are such that the plasma deposits an outer layer of soot and consolidates the soot and, for step (e), also consolidates the outer soot layer deposited at step (d), into an outer vitreous silica layer in accordance with the CFOT chemicals. Further, substantially simultaneous to the deposition and consolidation of the outer vitreous silica layer, the plasma effects depositing of inner soot particles and concurrent consolidation of same and consolidation of the inner soot layer deposited at step (d) into an inner layer of vitreous silica selectively doped in accordance with the CFIT chemicals.
A method according to the second aspect of the invention may perform step (d) by, for example, traversing the plasma at a second reverse traversal rate TR2 in a direction opposite the first direction while injecting CFIT chemicals into the void to flow in the first direction and injecting CFOT chemicals into the plasma, where the plasma, the CFIT chemicals, the CFOT chemicals and the second reverse traversal rate are such that an inner soot layer in accordance with the CFIT chemicals and an outer soot layer in accordance with the CFOT are deposited without substantial consolidation.
A third aspect of the invention is a method for making an optical fiber preform including steps of: (a) providing a silica tubular member, the silica tubular member having an interior surface surrounding a cylindrical void extending along a center axis and having an exterior surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) depositing an outer silica soot layer on the outer surface of the tubular member concurrent with depositing an inner soot layer inward on the inner surface of the tubular member, (d) concurrently consolidating the outer soot layer and the inner soot layer deposited at step (c) into, respectively, an outer vitreous silica layer and an inner vitreous silica layer, and (f) repeating steps (d) and (e) until a predetermined thickness of vitreous silica layers is formed by the steps.
A method according to the third aspect of the invention performs step (c) by, for example, traversing a plasma flame in a first direction along the rotating tubular member at a third forward traversal rate TF3 while injecting CFIT chemicals into the cylindrical void to flow in the first direction and injecting CFOT chemicals into the plasma. The generation of the plasma and the third traversal rate are such that the plasma deposits the outer layer of soot in accordance with the CFOT chemicals and, concurrently, effects deposition of the inner layer of soot in accordance with the CFIT chemicals.
Step (d) of a method according to the third aspect of the invention may be performed by, for example, traversing the plasma at a third reverse traversal rate TR3 in a direction opposite the first direction, preferably without injecting substantive CFOT chemicals into the plasma and without injecting substantive CFIT into the void. The third reverse traversal rate TR3 is set such that the inner soot layer and the outer soot layer deposited at step (c) are consolidated into, respectively, the inner vitreous silica layer and the outer vitreous silica layer.
A fourth aspect of the invention has similarity to the second aspect, but differs with respect to the steps of deposition and consolidation of silica on the inner surface of the tube. The fourth aspect of the invention is a method for making an optical fiber preform including steps of: (a) providing a silica tubular member, the silica tubular member having an inner surface surrounding a cylindrical void extending along a center axis and having an outer surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) depositing an outer soot layer on the outer surface of the tubular member concurrent with depositing an inner soot layer of substantially pure silica on the inner surface of the tubular member, (d) depositing a second outer soot layer on the outer surface of the tubular member concurrent with consolidating the deposited second outer soot layer and the outer soot layer deposited at step (c) into an outer vitreous silica layer, concurrent with injecting dopant chemicals into the cylindrical void and consolidating the dopant chemicals and the inner substantially pure silica soot layer into an inner doped vitreous silica layer, and (e) repeating steps (c) and (d) until a predetermined thickness of outer vitreous silica layers and inner doped vitreous silica layers are formed.
A method according to this fourth aspect of the invention performs step (c) by, for example, injecting CFIT chemicals into the cylindrical void to flow in a flow direction and, while the CFIT chemicals are flowing, traversing a plasma flame along the rotating tubular member at a fourth reverse traversal rate TR4 in a direction opposite the flow direction injecting CFOT chemicals into the plasma. The CFIT chemicals promote formation of the substantially pure silica soot. In this example, the generating of the plasma and the fourth reverse traversal rate are such that the plasma deposits the outer layer of soot in accordance with the CFOT chemicals and, concurrently, effects deposition of the inner layer of substantially pure silica soot in accordance with the CFIT chemicals.
A method according to the fourth aspect of the invention performs step (d) by, for example, traversing the plasma at a fourth forward traversal rate TF4 in the flow direction while injecting CFOT chemicals into the plasma, while also injecting CFIT into the void, where the CFIT chemicals include dopants for modifying the index of refraction of the substantially pure silica inner soot layer deposited at step (c) without effecting formation of additional silica soot. The fourth forward traversal rate and the CFIT chemicals flowing during step (d), for this example performance of the step, are such that the dopants for modifying the index of refraction of the substantially pure silica inner soot layer deposited at step (c) are consolidated with the substantially pure silica soot layer to form an inner vitreous silica layer doped in accordance with the CFIT chemicals, without effecting formation of additional silica soot.
A fifth aspect of the invention has similarity to the first aspect, but differs with respect to steps of deposition and consolidation of silica on the outer surface of the tube. The fifth aspect of the invention is a method for making an optical fiber preform including steps of: (a) providing a silica tubular member, the silica tubular member having an inner surface surrounding a cylindrical void extending along a center axis and having an outer surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) depositing an outer soot layer of substantially pure silica on the outer surface of the tubular member concurrent with depositing an inner layer of silica soot on the inner surface of the tubular member and consolidating the deposited soot into an inner layer of vitreous silica, (d) consolidating the outer layer of substantially pure silica soot deposited at step (c) into an outer layer of substantially pure vitreous silica, and (e) repeating steps (c) and (d) until a predetermined thickness of outer vitreous silica layers and inner vitreous silica layers are formed.
A method according to the fifth aspect of the invention performs step (c) by, for example, traversing a plasma flame in a first direction along the rotating tubular member at a fifth forward traversal rate TF5 while injecting CFIT chemicals into the cylindrical void to flow in the first direction and injecting CFOT chemicals into the plasma. The CFOT chemicals are preferably selected to promote formation of the substantially pure silica soot. The generating of the plasma and the fifth forward traversal rate TF5 are such that the plasma deposits the outer layer of substantially pure silica soot in accordance with the CFOT chemicals and, concurrently, effects deposition of the inner layer of silica soot in accordance with the CFIT chemicals.
A method according to the fifth aspect performs step (d) by, for example, traversing the plasma at a fifth reverse traversal rate TR5 in a direction opposite the first direction without injecting CFIT chemicals promoting additional formation of soot, deposited at step (c) without effecting formation of additional silica soot. The fifth reverse traversal rate consolidates the outside layer of substantially pure silica soot deposited at step (c) into an outer layer of substantially pure vitreous silica.
A variation of the first through fifth aspects of the invention includes a step of collapsing the tubular member into a preform. An example of this variation performs the collapsing step by generating a plasma with a predetermined collapsing temperature profile relative to the outer surface of the tubular member, and repeatedly traversing the plasma torch along the length of the tubular member.
A variation of the collapsing step includes periodically adjusting the temperature profile and the pressure differential between the inner and outer surfaces of the tubular member until the tubular member has collapsed into a preform.
Another variation of the collapsing step performs, concurrent with at least a portion of the repeated traversing of the plasma torch along the length of the tubular member collapsing, a step of forming additional vitreous silica layers above the outside surface of the tubular member. An example of this variation performs the forming of additional vitreous silica layers by injecting CFOT chemicals into the plasma as it is repeatedly traversed.
Still another variation of the first through fifth aspects of the invention, with or without the other variations, includes a further step of forming additional vitreous silica layers on the preform, by POVD, after the completion of the collapsing step.
Still another variation of the first through fifth aspects of the invention, with or without one or more of the other variations, is performing a step concurrent with or interspersed with one or more of the deposition or consolidation steps of measuring a distance from a reference point relative to the plasma torch and the outside surface of the tubular member and moving the plasma torch based on the sensed distance to maintain a predetermined spacing between the reference point and the outer surface.
A further aspect of the invention is a method for making a preform comprising steps of (a) providing a silica tubular member, the silica tubular member having an interior cylindrical surface surrounding a cylindrical void extending along a center axis and having an exterior cylindrical surface coaxial with the center axis, (b) rotating the silica tubular member about the center axis, (c) forming an outer silica layer along a length of the tube above the outer surface of the tube concurrent with depositing an inner soot layer along the length of the tube inward from the inner surface of the tube, (d) repeating step (c) until a predetermined thickness of outer vitreous silica layers or inner soot layers is formed by the step, (e) drying the tubular member with the formed outer vitreous silica layers and the deposited soot layers, (f) consolidating the deposited inner soot layers, and (e) collapsing the tubular member into a preform.
A variation of the first through sixth aspects of the invention, with or without one or more of the other variations, uses a lower quality starting tube that can be etched away by a plasma flame. This etching process traverses a plasma flame which also heats the inside of the tube. The plasma heating of the inside of the tube causes, concurrent with the etching, deposition and/or consolidation of soot on the inside of the tube.
An example method according to this variation has two steps. The first step repeatedly traverses the plasma to etch the tube from the outside and, preferably concurrently, to deposit silica soot and/or consolidate silica soot into vitreous silica on the inside of the tube. The first step is complete when the starting tube is etched away, leaving a new tube consisting of the vitreous silica consolidated on the inside of the starting tube. The second step performs the concurrent inside/outside deposition and/or deposition/consolidation performed by the previously described methods according to this invention, using the new tube as the starting tube. The first step may deposit the inner soot, and consolidate the soot in the same manner used by the second step to deposit and consolidate the inner soot. Alternatively, the first step may deposit and/or consolidate the soot on the inner surface using a method different from that used by the second step to deposit and/or consolidate soot on the inner surface.
An example first step according to this variation includes a step (A), which traverses the plasma flame along the starting tube while injecting CFIT chemicals into the void. The traversing is at a rate E1. The rate E1 is such that the plasma flame etches the outside surface of the starter tube and causes silica soot particles to be deposited downstream of the hot zone within the tube, and consolidates the soot particles into a vitreous silica layer. At the completion of step (A), the starter tube is thinner, and there is a thin new tube of vitreous silica formed on the inside of the starter tube. Step (A) is repeated until the starting tube is etched away. The new tube consists of the repeated layers of vitreous silica deposited and consolidated as the plasma was traversed to etch the starter tube away.
In view of the above-identified and other shortcomings in the prior art, an object of the present invention is an apparatus and method for high rate production of optical fiber preforms.
Another object of the invention is an apparatus and method for high rate production of large diameter optical preforms.
Still another object of this invention is a method and apparatus for high rate production of optical fiber preforms eliminating one or more of the steps of consolidation, sintering, and jacketing used in the methods known in the art.
Another object of this invention is a method for increasing the incorporation of dopants inside a tubular member, together with increasing the processing rate.
These and other objects, features and advantages of the present invention will become more apparent to, and better understood by, those skilled in the relevant art from the following more detailed description of the preferred embodiments of the invention taken with reference to the accompanying drawings, in which like features are identified by like reference numerals.